1. Field of the Invention
The invention relates to devices and circuits for "non-contacting" measurement of current, whose output is an electrical signal which is isolated electrically from the conductor whose current is being sensed; and more particularly to such a sensor which does not require complex temperature compensation or expensive, critical semiconductor or magnetic components.
A very simple device of this type is the so-called current transformer, which has a transformer core with a line current winding and a secondary winding. The secondary winding is terminated with a low value resistance, so that the impedance seen from the line is very small. Usually the primary has only a few turns, or may simply consist of an insulated length of line conductor passing through a core opening and functioning as one turn. Providing a sufficient insulation of the primary coil so that breakdown voltage is high, and leakage currents between primary and secondary are negligibly small, is fairly easy; and such a transformer is relatively unaffected by temperature extremes, is extremely stable, and has a very accurately known current ratio so long as the flux level in the core does not approach saturation. However, after an initial transient, DC components in the line current are not mirrored by a corresponding component in the secondary; measurement of very low frequencies requires a large core; and relatively low values of direct current components in the line current (of the order of magnitude of the peak of typical exciting current) will cause the core to saturate sufficiently for at least a part of each AC cycle so that even the AC components are no longer accurately mirrored by the voltage across the secondary winding.
If a current sensor is to be used as a trip signal source in a circuit breaker, so that ratings are easily changed, it is desirable that the sensor operate down to DC even when the power source is AC. It further is desirable that an electronic trip breaker can be rated for DC operation, and accurate measurement of DC is then mandatory. For such applications, or any in which DC or very low frequencies are involved, other types of non-contacting current sensor are then required.
2. Description of the Prior Art
A known type of current sensor, which is operable down to DC, uses Hall Effect sensors. However, these devices are relatively temperature sensitive and position sensitive, and can require expensive calibration if high accuracy is required, such as when comparing currents in two different conductors.
A still more recent development involves an active current-balancing circuit, which uses a transformer core which is driven between saturation flux values by a high frequency alternating current. Such a device is described by K. Harada and H. Sakamoto in "Current Sensors with a Small Saturable Core and Mosfets", IEEE Transactions on Magnetics, vol. 24, no. 6 (November 1988). This device balances the effect of a current in a line coil with a bucking current flowing in an opposite direction through a bucking coil and through a measuring resistor. The bucking current has a value which is exactly related to the line current by the turns ratio of these coils, down to DC. In addition, a high frequency magnetizing/switching current flows through the bucking coil and measuring resistor in response to the alternate application of opposite polarity voltages from a switching circuit.
The circuit described in this article uses a Permalloy 80 core, which has high permeability up to a certain flux, and then exhibits a very sharp saturation. A secondary coil provides a trigger voltage to the switching circuit, which is a so-called transistor core multivibrator.
Two complementary field effect transistors have their source terminals connected to one end of the bucking coil and their drain terminals respectively connected to positive and negative power supplies. Their gate terminals are connected together and to the secondary coil to cause the transistors to conduct alternately. When one transistor is conducting, a constant voltage is applied to the bucking winding, and flux and magnetizing current through the bucking coil rise linearly. The secondary coil is connected such that the voltage applied to the gate, due to the linearly rising flux in the core, holds the transistor on until the core saturates. Upon core saturation there is a sudden increase, or spike, in the current through that transistor. Because of the circuit design and the inter-terminal capacitances of the transistors, the current spike causes the FET's to be switched, cutting off the one transistor which had been conducting.
Upon switching, current flow cannot stop instantaneously, so it continues through the reverse diode associated with the other transistor. The spike current falls rapidly until the core is no longer saturated. The magnetizing current then continues to fall linearly to zero, and rises linearly in the opposite direction through the other transistor. When the core reaches opposite saturation, and there is an opposite current spike, switching occurs again. Thus, a very small generally triangular current, with large spikes at each peak, flows at a switching frequency determined by the core size and characteristics, the number of bucking coil turns, and the supply voltages.
When the line current is negligible, this high frequency current is the only current component flowing through the bucking coil, and transistor switching is symmetrical. When the line current is substantial, the voltage across the measuring resistor due to the measuring current affects the speed of the generally linear changes of current as a function of the polarity of the change, so that transistor switching becomes repeatably asymmetric.
The Harada/Sakamoto circuit has the advantage that it is effective down to DC. However, line current components at frequencies which are a significant fraction of the switching frequency cannot be accurately detected because of the filtering which is required to reduce the effect of the current spikes on any circuit connected to receive the voltage across the measuring resistor. The large current spikes are, themselves, a potential source of electromagnetic interference with other circuits or systems. In addition, the circuit cost is undesirably high because of the need for a sharply saturating core which must be wound from metal tape.